The requirements for increasingly high areal recording density impose increasingly greater demands on thin film magnetic recording media in terms of remanent coercivity (Hr or Hcr), magnetic remanance (Mr), coercivity squareness (S*), medium noise, i.e., signal-to-noise ratio (SNR), and narrow track recording performance. It is extremely difficult to produce a magnetic recording medium satisfying such demanding requirements.
The linear recording density can be increased by increasing the Hr of the magnetic recording medium. However, this objective can only be accomplished by decreasing the medium noise, as by maintaining very fine magnetically non-coupled grains. Medium noise is a dominant factor restricting increased recording density of high density magnetic hard disk drives. Medium noise in thin films is attributed primarily to inhomogeneous grain size and intergranular exchange coupling. Accordingly, in order to increase linear density, medium noise must be minimized by suitable microstructure control.
A conventional longitudinal recording disk medium is depicted in FIG. 1 and comprises a substrate 10, typically an aluminum (Al)-alloy, such as an Al-magnesium (AlMg) alloy plated with a layer of amorphous nickel--phosphorus (NiP). Alternative substrates include glass, ceramic and glass-ceramic materials, as well as graphite. There are typically sequentially sputter deposited on each side of substrate 10 an underlayer 11, 11', such as Cr or a Cr alloy, a magnetic layer 12, 12', such as a cobalt (Co)-based alloy, and a protective overcoat 13, 13', such as a carbon-containing overcoat. Typically, although not shown for illustrative convenience, a lubricant topcoat is applied on the protective overcoat 13, 13'.
It is recognized that the magnetic properties, such as Hr, Mr, S* and SNR, which are critical to the performance of a magnetic alloy film, depend primarily upon the microstructure of the magnetic layer which, in turn, is influenced by the underlying layers, such as the underlayer. It is also recognized that underlayers having a fine grain structure are highly desirable, particularly for growing fine grains of hexagonal close packed (HCP) Co alloys deposited thereon.
Conventional practices in manufacturing magnetic recording media comprise Direct Current (DC) magnetron sputtering and high temperatures in order to obtain Cr segregation in Co-alloy grain boundaries to achieve high Hr and high SNR. Conventional practices, therefore, employ a high substrate heating temperature, e.g. above about 200.degree. C., e.g. about 230.degree. C. to about 260.degree. C., in order to achieve a desirably high Hr. However, such high substrate heating temperatures result in a reduced S* and, hence, increased medium noise. In order to increase information storage capacity, recording media with higher Hr and lower medium noise are manifestly required. Higher Hr narrows the pulse width, thereby enabling reduction of the bit length for higher recording density, while lower media noise leads to higher SNR.
In order to increase Hr, magnetic alloys containing platinum (Pt), such as Co--Cr--Pt--tantalum (Ta) alloys have been employed. Although Pt enhances film Hr, it was found that Pt also increases media noise. Accordingly, it has become increasingly difficult to achieve high areal recording density while simultaneously achieving high SNR and high Hr.
As media noise predominately stems from exchange and magnetostatic interactions among magnetic grains, SNR can be improved by minimizing such interactions. For example, such interactions can be suppressed by separating or segregating the magnetic grains either physically or chemically. Prior efforts in this area, however, have dealt with relatively low Hr media, e.g. less than about 2,000 Oe. Little effort, to date, has been devoted to increasing Hr and simultaneously reducing media noise for high areal recording density medium.
Conventional practices in manufacturing magnetic recording media comprise depositing a Cr underlayer, particularly for thin film media for longitudinal recording. The Cr underlayer provides a favorably crystallographic structure, such that the magnetic alloy layer subsequently deposited thereon, such as a Co-alloy layer, can nucleate with the c-axis of the HCP structure in-plane on virtually in-plane via grain to grain epitaxy. However, as the drive for higher and higher areal recording density increases and film thickness decreases, it becomes increasingly difficult to satisfactorily reduce the grain size of the magnetic layer and to decrease intergranular coupling in order to achieve satisfactory media noise reduction.
Accordingly, there exist a need for high areal density magnetic recording media exhibiting a high Hr and a high SNR.